Selective absorbers



Jan. 13,1959 w. L. PRITCHARD ET AL 2,869,085

SELECTIVE ABSORBERS 2 Sheets-Sheet 2 Filed Jan. 19, 1954 /Nl/ENTOR W/LBUR L. PR/TCHARD AgMAsA PRATT ATTORNEY u to that of the main guide.

Other and further advantages of this invention will be.

, United States Patent SELECTIVE ABSORBERS .Wilbur L. Pritchard, Water-town, Mass., and Amasa Pratt,

Sherman Oaks, Calif., assignors to Raytheon Manufacturing Company, Waltham, Mass., a corporation of Delaware Application January 19, 1954, Serial No. 404,828 2 Claims. (Cl. 333-98) This invention relates to means for absorbing radio frequency energy at a certain frequency propagating at a higher mode than the desired mode while permitting this energy to be propagated along a transmission line in the lower mode.

When it is required to transmit radio frequency energy through a wave guide or other transmission line over considerable distances, it is frequently desirable to use a size of guide that is larger than required for the wave length of the energy to be propagated in order to take advantage of the lower attenuation of the larger wave guide. However, it is possible, if the larger wave guide is sufiiciently large, for energy at the desired frequency to be propagated in the larger wave guide in the next higher mode, such as the TE mode. Energy in this higher. mode can be generated by the presence of a discontinuity in the wave guide, such as a bend in the H plane of the wave guide. Under certain conditions, the line may become resonant to energy propagated at this higher mode and build up sufi'icient voltage to break down. This is particularly likely to happen if the larger guide is joined to the. smaller guide at both ends, in which case the energy generated at the higher mode in the larger guide cannot be propagated in the sections of smaller guide and is reflected from the junction points to build up in the larger guide. Consequently, it is desirable to absorb any ene rgy generated in the larger. guide in the higher mode without appreciably attenuating the energy at the lower {mode This is accomplished in the present invention by coupling an auxiliary wave guide so dimensioned as to propagate energy at the desired frequency in the T E mode and so connected to the main guide as to permit only energy propagating in the main guide in the TE mode to be coupled into the a uxiliary guide. Resistive terminations in the auxiliary wave guide absorb the energy propagating in it. In one embodiment, the auxiliary wave guide is mounted with its axis parallel to that of the main guide and coupled to it through an opening inthe wall of the main guide. In another embodiment the axis of the auxiliary guide is perpendicular apparent as the description thereof proceeds, reference being had to the accompanying drawingswherein'z Fig. 1 is a schematic diagram of one embodiment of the invention;

Fig. 2 is a transverse section of the embodiment shown in Fig. 1 showing the electrical vectors of the TE mode;

Fig. 3 is a graph of the variation of voltage with distance across the guide for the TE mode;

Fig. 4 is a transverse section of the embodiment shown in Fig. 1 showing the electrical vectors of the TE mode;

Fig. 5 is a graph of the variation of voltage with distance across the guide for the TE mode;

Fig. 6 is a schematic diagram of a second embodiment of the invention;

Fig. 7 is a transverse section of the embodiment shown in Fig. 6 showing the electrical vectors of the TE mode;

2,869,085 Patented Jan. 13, 1959 of wave guide dimensioned so that energy at the desired 10.

frequency can be propagated along it in either the TE or the TE mode. The energy propagated in the TE mode has a single maximum of the electrical vector, as

shown by the concentration of the arrows 11 representing the electrical vectors in Fig. 2 and the single maximum of the graph 12 in Fig. 3 representing voltage distribution across the guide. TE mode has two maxima of the electrical vectors, as

Any energy propagated in the shown by the two regions of concentration of the arrows 13 representing the electrical vectors in Fig. 4 and the two maxima of the graph 14 in Fig. 5 representing voltage distribution across the guide.

An auxiliary section of wave guide 15 is mounted with its axis parallel to that of the main guide 10. The guide 15 is dimensioned so as to propagate energy at the operating frequency in the TE mode only. A wall 16 of this guide parallel to the electrical vector of energy propagated in this manner, as indicated by the arrows 17 in Fig. 4, is in common with a wall 18 of the guide 10 and is perpendicular to the electrical vector of energy propagated in the guide 10, as indicated by the arrows 13. A slot 20 is formed in this common wall 16. The auxiliary guide 15 is terminated at each end by a block 21 of lossy dielectric material.

In operation, energy at the operating frequency propagated along the wave guide 10 inthe TE mode produces a voltage maximum at the slot 20, as indicated by the greater concentration of the arrows: 11 in this region.

The voltage maximum is shown by the graph 12 in which the distance along the horizontal axis 22 represents the distance across the guide, and the distance. along. the vertical axis 23 represents the amplitude of the voltage at diflerent points across the wave guide 10. The amplitudes of the two vectors at the edges of the slot 20 are represented by the arrows 24 and 25. Itwill be seen that there is little change voltage in the region of the slot, that is Y r r oind AEQO dx Consequently, little or no energy is propagated in this mode through the slot 20 intothe guide 15.

However, in the case of energy at the. operating fre quency propagated in guide 10 in the TE mode, as

shown by the arrows 13 shown in Fig. 4, the vectors are in opposite phase on opposite sides of the slot 20, as shown by the arrows 26 and 27, and the voltage varies rapidly in thisfregion, as shown by :the graph 14 which has its maximum slope in this region and its maximum ditference, AB, in the same region. The result is that considerable energy propagating in the TE- mode in the main guide 10 is coupled through the slot 20 to the auxiliary guide 15 where it is propagated in the TE mode, as shown by the arrows 17. This energy is propagated in both directions towards the ends of the guide 15, where it is absorbed by the blocks 21 of lossy dielectric material. Thus any energy developed in the main guide at the harmonic of the desired frequency is coupled into the auxiliary guide and dissipated without appreciable loss of energy propagating in the useful TE mode.

A second embodiment is illustrated in Figs. 6 through 9. In this embodimennreference numeral 30 designates a section of wave' guide dimensioned so that energy at the desired frequency can be propagated along the guide in the TE and T E modes. The energy propagated in the TE mode, as shown by the arrows 31 in Fig. 7 representing the electrical vectors, reaches a maximum at the center of the guide 30, as shown by the graph 32 in Fig. 8.

Any energy at the operating frequency propagated in the TE or higher modes, as shownby the arrows 33 in Fig. 9, reaches two maxima of opposite'polarity, as shown bythe graph 34 of Fig. 10'.

An auxiliary section of wave guide 35 is mounted with its axis perpendicular to that of the main guide 30. The guide 35 is dimensioned to propagate the energy at the desired frequency in the TE mode. The guide 35 is joined to the guide 30 at an opening 36 in a wall 37 of the guide 30 that is perpendicular to the electrical vector 33. The guide 35 is also terminated by a block of lossy dielectric material 38.

In operation, energy at the operating frequency propagated along the wave guide 30 in the TE mode produces a voltage maximum at the opening 36, as indicated by the greater concentration of the arrows 31 in this region. The voltage maximum is shown by the region 40 on the graph 32 in which the distance x along the horizontal 'axis 41 represents the distance across the guide, and the distance along the vertical axis 42 represents the amplitude of the voltage at different points across the wave guide 30. The amplitude of the two vectors at the edges of the opening 36 are represented by the arrows 44 and 45. It will be seen that thereis little change in voltage in the region of the opening that is,

%Qi and AEZO Consequently, little or no energy propagating in this mode in guide 30 is coupled through the opening 36 into the guide 35.

However,.as before, the energy at the operating frequency propagating in guide 30 in the TE mode, as shown by the arrows 33 in Fig. 9, has its-electrical vectors in opposite phase-on opposite sides of the opening 36, as shown by the arrows 46 and 47, and the voltage varies rapidly in this region 48, as shown by the graph 34 in Fig. 10 which has its maximum slope 4 guide in the TE mode is coupled into the auxiliary guide 35 and dissipated without appreciable'loss of energy at the operating frequency propagating in the main guide 30 in the useful TE mode. The embodiments shown disclose the use of rectangular wave guide in carrying out the invention. However, other forms of transmission line, such as coaxial cable and circular wave guide, can also be used.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. it is, accordingly, desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. In a selective absorber for radio frequency energy, the combination of a section of wave guide of uniform cross section dimensioned to propagate energy at a desired frequency in a first mode and a second mode, a second section of a wave guide dimensioned to propagate radio frequency energy at the desired frequency in the first mode but not in the second mode, and means to couple energy propagated in the first section of wave guide in the second mode but not that propagated in the first mode from the first section of wave guide into the second section of wave guide comprising a wall common to both wave guides and perpendicular to the electrical vector of energy propagating in the first wave guide and parallel to the electrical vector of energy propagated in said second wave guide, and a slot formed in this common wall.

2. In a selective absorber for radio frequency energy, the combination of a section of wave guide of uniform cross section dimensioned to propagate energy at a de- 5 sired frequency in a first mode, a second section of wave guide dimensioned to propagate radio frequency energy at the desired frequency in the first mode, but not in the second mode, said second section of wave guide being terminated with lossy material, and means to couple energy propagated in the first section of wave guide in the second mode'but not that propagated in the first mode from the first section of wave guide into the second section of wave guide comprising a Wall common to both wave guides and perpendicular to the electrical vector r of energy propagating in the first wave guide and parallel to the electrical vector of energy propagating in said second wave guide, and a slot formed in said common wall.

citer, Proc. of the I. R. B, vol. 37, No. 9, September 1949.1) 1031. p 

